How to scale this object (a Tail of an Asteroid), so it is not crooked?
I found something crucial, your models always have to be Applied on Scale and Rotation, specially if they are used in animation. This may be the source of your problem when you scale the geometry down, when you are modelling always remember to Apply Rotation &amp; Scale to your models, it's a good guideline to keep. (CTRL+A+Apply Rotation &amp; Scale in Object Mode)


Now, for the trick you wanted it's really a dirt trick, it's something harmful you can do, but it reproduces the effect you want, plus you can animate it. I selected your root bone of the asteroid.


And scaled it down while locking the Axis Z, (S+Shift Z)


It's a ugly solution and messing with bone's scale it's a bad way of doing things, but, it works. So for your animation, maybe you can animate the X and Y scale of your root bone.